CIS based thin-film solar cells were regarded as practically usable extensively. Patent document 1 among many publications discloses that to deposit a cadmium sulfide (CdS) layer as a high-resistance buffer layer on a light absorption layer comprising a thin CuInSe2 film is necessary for obtaining a thin solar cell having a high conversion efficiency. There is a statement in patent document 1 to the effect that the solution deposition method described therein, in which a thin CdS film is deposited from a solution, has the following effect. The immersion of a thin CuInSe2 light absorption layer in the solution not only forms a heterojunction interface but also produces the effect of etching or selectively cleaning the surface of the thin light absorption layer to thereby form a high-quality heterojunction with the thin CuInSe2 light absorption layer and heighten shunt resistance. In the case where a cadmium sulfide (CdS) layer is deposited as the high-resistance buffer layer, efforts are being made to minimize the discharge of a waste liquid containing cadmium, which is highly toxic. However, solid CdS and an alkaline waste liquid are yielded in large quantities and, hence, the cost of waste treatments has been an obstacle to a reduction in solar cell production cost.
On the other hand, patent document 2 discloses a production process in which a zinc compound crystal containing oxygen, sulfur, and hydroxyl groups, i.e., Zn(O,S,OH)x, chemically deposited from a solution on a p-type light absorption layer is used as a high-resistance buffer layer, whereby a thin solar cell having the same high conversion efficiency as in the case of using a CdS layer as a buffer layer can be obtained. The production process disclosed in patent document 2 is effective in eliminating the use of a CdS buffer layer, which is considered to be essential to processes for producing a thin solar cell having a high conversion efficiency. However, a solid ZnO/ZnS mixture and an alkaline waste liquid are yielded in large quantities in this case also, and the cost of waste treatments has been an obstacle to a reduction in solar cell production cost.    Patent Document 1: U.S. Pat. No. 4,611,091 (Class 136/260; issued on Sep. 9, 1986)    Patent Document 2: Japanese Patent No. 3249342 (JP-A-8-330614)